Field of the Invention
This invention relates to implantable medical devices, such as stents, that have coatings that control erosion of bioabsorbable substrates of the devices.
Description of the State of the Art
This invention relates generally to implantable medical devices having a range of mechanical and therapeutic requirements during use. In particular, the invention relates to radially expandable endoprostheses that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty or valvuloplasty) with apparent success.
Plaques have been associated with stenosis and restenosis. While treatments of plaque-induced stenosis and restenosis have advanced significantly over the last few decades, the morbidity and mortality associated with vascular plaques have remained significant. Recent work suggests that plaque may generally fall into one of two different general types: standard stenotic plaques and vulnerable plaques. Stenotic plaque, which is sometimes referred to as thrombosis-resistant plaque, can generally be treated effectively by the known intravascular lumen opening techniques. Although plaques induce stenosis, these atherosclerotic plaques themselves are often a benign and are an effectively treatable disease.
Unfortunately, as plaque matures, narrowing of a blood vessel by a proliferation of smooth muscle cells, matrix synthesis, and lipid accumulation may result in formation of a plaque which is quite different than a standard stenotic plaque. Such atherosclerotic plaque becomes thrombosis-prone, and can be highly dangerous. This thrombosis-prone or vulnerable plaque may be a frequent cause of acute coronary syndrome. Both restenosis and vulnerable plaque may be treated by administering to a patient an active agent or a suitable combination of active agents through the use of an implantable medical device, such as a stent.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent, through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand.
Stents have been made of many materials including metals and polymers. Polymer materials include both biostable and biodegradable polymer materials. Metallic stents are typically formed from biostable metals. Bioerodible metal stents have been described previously. U.S. Pat. No. 6,287,332 B1 to Bolz et al., U.S. Pat. Appl. Pub. Ser. No. 2002/0004060 A1 to Heublein et. al.
The stent must be able to satisfy several mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. This requires a sufficient degree of strength and rigidity or stiffness. In addition to having adequate radial strength or more accurately, hoop strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Therefore, a stent must be capable of exhibiting relatively high toughness which corresponds to high strength and rigidity, as well as flexibility.
Furthermore, it may be desirable for a stent to be biodegradable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodible materials such as bioabsorbable polymers should be configured to completely erode only after the clinical need for them has ended. In addition, a stent should also be capable of satisfying the mechanical requirements discussed above during the desired treatment time.
In general, it is desired that a biodegradable stent maintain its mechanical stability during a desired treatment period. However, some erodible metals degrade much faster than a desired treatment time. In addition, if a stent erodes too quickly, large pieces of the stent may detach from the eroding stent and cause embolization in a vessel. In addition, polymers that exhibit a high degree of bulk eroding behavior can experience a substantial deterioration in mechanical properties that could lead to failure prior to the end of the treatment period. Therefore, there is a need to control erosion of biodegradable stents to maintain structural stability.